The present invention relates to a toner image fixing device which has a novel construction for thermally fixing a toner image developed on a sheet-like carrier, e.g., a recording paper.
In a conventional image-recording device, a recording medium forms thereon a toner image and transfers it onto a common paper sheet which is subjected to fixing the toner image thereon and then is delivered out of the device. Generally, the toner image is fixed on a paper sheet by fusing.
A fixing device used in the image recording device comprises a rotatably mounted heating roller made of aluminum drum coated with fluorocarbon resin (e.g., PTFE: polytetrafluoroethylene sold under the trade name "Teflon") which is free from adhesion of toner, a pressure roller coated with silicone rubber disposed as pressed against the heating roller to form therebetween a contact portion (nip) utilizing elastic deformation of the pressure roller. While a sheet carrying a toner image developed with toner thereon passes through the printing nip, the toner image is heated and fixed by fusing to the sheet. A heater consisting of, e.g., a halogen lamp is mounted in the heating roller to heat the latter at a specified temperature necessary for fusing toner of the toner image on the sheet.
In the above-mentioned fixing device, the heating roller is made of an aluminum drum having wall thickness of 1.0 mm to several millimeters and outer diameter of 20 to 60 mm. A toner image developed with toner on the paper sheet is fixed by fusing while the sheet passes through a nip (contacting portion) between the pressure roller and the heating roller heated at a specified temperature by the heater axially mounted therein.
Another example of a toner image fixing device that is different from the above-mentioned device is proposed, wherein a belt being an endless film of 10 to 50 microns in thickness made of heat-resistant material (e.g., polyamide) envelops therein a heater supporting member which supports a heater having a resistance on a ceramic substrate in such a manner that the heater may be in contact with an internal surface of the endless belt. A pressure roller is disposed opposite to the heater and pressed against an external surface of the endless belt to form a nip portion through which a paper sheet having a developed toner image passes being subjected to fixture of the toner image thereon by fusing.
As described previously, the conventional toner image fixing device uses the heating roller having a thick wall of 1.0 to several millimeters in radial direction, which, therefore, shall be previously heated by conduction heat to a specified working temperature of its surface for a warm-up time of several seconds to several minutes. The long warm-up time of the heating roller degrades the controllability of the device as well as increases the power consumption. To put a paper sheet into contact with the heating roller, it is necessary to use the pressure roller having a metal core covered with silicone rubber, which is expensive in itself and increases a manufacturing cost of the conventional device.
In comparison with the above-mentioned device, the other conventional device uses a thin-film type belt to be heated and, therefore, can save its warm-up time and reduce power consumption required. However, this device also has to use the pressure roller for putting the toner-image carrying paper sheet into close contact with the rotating endless belt, that irrevocably leads to increasing the manufacturing cost of the device. The device must be provided with means for driving the endless belt, (e.g., a driving roller and a driven roller), that may not only complicate the construction of the device but also increase its manufacturing cost.